It is known to provide ventilators at the upper ends of conduits in order to enhance the vertical flow through them. Such ventilators can be employed for enhancing the draft in a flue from a fireplace or furnace, or for enhancing the flow in a ventilating conduit. In some ventilating systems it is desired to place the suction fan at the upper end of the exhaust conduit to remove it as far as possible from the location where the air is being drawn from, so as to minimize noise. Furthermore it is frequently desired to enhance the draw at the outlet end of a conduit in order to prevent leakage out of the conduit, as in an installation for exhausting noxious fumes or industrial gases.
The simplest type of such a ventilator is a so-called static ventilator which is so constituted as to employ normally present air currents to enhance the draft from the upwardly open conduit mouth. This can most simply be done by mounting a flat plate above the upper mouth of the conduit so that as air passes horizontally between the plate and the conduit mouth gases are sucked from the conduit by the Pitot effect. Such a system is almost completely ineffective when the draft past the conduit mouth is not horizontal, and can indeed allow outside air to blow back down the stack.
Dynamic ventilators are known, such as described in my copending application Ser. No. 869,222 filed Jan. 13, 1978 (now U.S. Pat. No. 4,200,035). These arrangements have a fan provided at the upper end of the conduit mouth and serving physically to suck air out of the conduit. Such arrangements are extremely effective, yet have several disadvantages. Mainly the disadvantage of these systems is that the air rising out of the conduit passes over the blades of the fan and normally also over the drive motor for the fan, thereby heating it considerably. As a result the service life of such a system, in particular in arrangements where corrosive vapors or the like are carried on the air rising from the conduit, is relatively short. Furthermore when the air rising from the conduit is quite hot this heat is transmitted to the fan and to the motor so that, once again, their service life is considerably reduced.